Unless of course you're one of those weirdos who plays classes you actually enjoy instead of going for MAX DAMAGE. But who would ever be that crazy?

People who dont like to be sunwelled.
People who like having the option to do something else beside DPS for a change if they get bored
People who dont like queues that last longer than a TV show.
Being able to switch to a healer in BGs to actually help your team win